1. Field of the Invention
The present invention relates to a torque detection device for a wave gearing. More specifically, the present invention relates to a torque detection device for the wave gearing that is capable of detecting transmitted torque with good precision with the aid of strain gauges attached on a flexible external gear, using the elastic deformation of the flexible external gear of the wave gearing.
2. Description of the Related Art
A wave gearing 1 comprises a rigid internal gear 2, a flexible external gear 3 disposed therein, and a wave generator 4 that flexes this flexible external gear 3 in the radial direction to partially engage the rigid internal gear 2 and moves the engaging positions in the circumferential direction, as shown in FIGS. 1A and 1B. Typically, the flexible external gear 3 is elliptically flexed by the elliptically contoured wave generator 4. When the wave generator 4 is rotated by a motor or other means, relative rotation is generated between the gears 3 and 4 due to the difference in number of teeth between these gears. The rigid internal gear 2 is commonly fixed, the flexible external gear 3 is used as an output element, and an output of reduced rotational speed is obtained from the external gear 3.
There have been known methods making use of the elastic deformation of the flexible external gear 3 in order to detect the output shaft torque of the wave gearing 1, namely, the torque transmitted through the flexible external gear 3. As shown in FIGS. 15A and 15B, the generally used, cup-shaped flexible external gear 3 comprises a flexible cylindrical body portion 12, a disk-shaped diaphragm 13 formed integrally at one end thereof, a boss 14 formed integrally on the center portion of the diaphragm 13, and external teeth 15 formed on the external circumference portion of the open end of the cylindrical body portion 12. Strain gauges are adhered on the diaphragm 13 or on the external circumferential surface of the cylindrical body portion 12, whereby the output shaft torque can be detected based on the outputs thereof.
Since the flexible external gear 3 is elliptically flexed by the wave generator 4, each portion of the flexible external gear 3 is repeatedly and forcedly deformed in the radial direction as the wave generator rotates. Thus, strain unrelated to the transmitted torque is generated in the flexible external gear 3. Each portion of the flexible external gear 3 reciprocates twice with a fixed amplitude in the radial direction with each rotation of the wave generator. Strain unrelated to the transmitted torque is therefore sinusoidal strain in which two periods per one rotation of the wave generator constitute the fundamental period (one period: 180°).
Conventionally, a perpendicular biaxial strain gauge f1(p) (R1, R2) and a similar perpendicular biaxial strain gauge f2(p) (R3, R4) are adhered on the surface of the cup-shaped flexible external gear 3, for example, the inner surface 13a of the diaphragm 13 in a matter mutually offset by 90°, as shown in FIG. 15B. The gauges are connected so as to constitute a Wheatstone bridge circuit, and the strain component of the fundamental period is cancelled based on the output of both strain gauges. However, the linearity of the detection output is inadequate with this method, and rotational ripple components that have short periods (a multiple of the fundamental period) remain in the detection output. The principal cause of rotational ripple is thought to be an asymmetry in the elliptical deformation of the flexible external gear.
To cancel the strain of the secondary component (period: 90°), efforts have been made to mutually offset by 45° four perpendicular biaxial strain gauges that are offset by 90° in relation to each other, for a total of eight strain gauges. To further improve the linearity of the detection output, a configuration has also been proposed whereby perpendicular biaxial strain gauges are symmetrically disposed within a range of 360°, and these are mutually wired to form a Wheatstone bridge. Examples of the mounting positions of the perpendicular biaxial strain gauges and configuration examples of Wheatstone bridge circuits are shown in FIGS. 16 to 19.
Torque detection devices for wave gearings that use strain gauges are disclosed, for example, in JP-A 9-184777 and JP-A 2000-131160 that were assigned to the same assignee of the present application.
However, the conventional torque detection devices for the wave gearings requires a large number of strain gauges that must be mounted on the flexible external gear. This causes to increase the manufacturing price of the strain gauge type detection device in comparison with a load cell or other detection devices.
In the case of a small-sized wave gearing, it is difficult to secure enough space for mounting a large number of perpendicular biaxial strain gauges. The wiring work for each strain gauge is also difficult.